1. Field of the Invention
The present invention relates to a method and apparatus for determining stalling of a procession of moving articles and, more particularly, to such a method and apparatus used in an induction sealing apparatus which seals a foil sheet or liner to the opening of a container.
2. Related Art
Induction sealing units for sealing, hermetically sealing or tamper-proof sealing a container with a foil liner are typically included in conveyor systems for high volume applications. These systems usually have flat or tunnel sealing heads mounted above a conveyor, which conveyor carries a plurality of containers to be sealed into proximity with the sealing head.
The containers to be sealed are preconditioned to include a foil liner disposed over the opening of the container. Usually, the foil liner is held in place by a screw-on or snap-on cap as is known in the art. Once the preconditioned container is brought within a predetermined distance from the sealing head, a coil within the sealing head produces an electromagnetic field near the foil liner which is disposed within the cap. The electromagnetic flux produced by the field causes current to flow in the foil liner which causes the foil liner to heat and melt the lip of the container (and wax compound if used). The molten plastic adheres the foil liner to the opening of the container and, when the plastic cools, a seal is produced. The downward force supplied by the cap ensures a proper bond between the foil liner and the opening of the container, particularly when the molten plastic is cooling.
Typically, the containers are moved past the sealing head in a continuous fashion. The timing of the movement of articles past the sealing head is such as to assure sufficient heating to effect sealing but not so long as to cause overheating which could damage the containers, the product therein, or cause a fire. Accordingly, it is necessary to assure that the containers move past the sealing head at the predetermined rate and to set off an alarm and/or disable the power to the sealing head if the movement of the procession of articles stalls for any reason.